PII YL UM PL A THELMINTHES 



111 



and may also be developed along the margins of the body. In 

 the triclads, a single pair of eyes is usually present, as is also 

 the case with the rhabdocoels, though these last may have two 

 pairs or even a single eye. The sense of touch is very highly 

 developed in the Turbcllaria and tentacles are 

 frequently found, but the entire body is also 

 highly sensitive, rendered so especially by the 

 presence of sensory hairs. 



Reproduction. — Except for some rhabdocoels, 

 the Turbellaria are hermaphroditic. The repro- 

 ductive organs differ considerably in the different 

 orders. In most of the fresh-water forms, the 

 eggs undergo a simple, direct development but 

 some of the polyclads have a more complicated 

 developmental cycle. The larval polyclad is 

 called a Miiller's larva. Asexual reproduction 

 by transverse fission occurs in some rhabdocoels, 

 frequently in the event of rapid fission, giving 

 rise to a complex chain of individuals. Such a 

 condition is shown in Fig. 62 in which the order 

 of formation of the successive individuals is 

 indicated. Similar chains are potentially present 

 in the planarians, which undergo fission as shown 

 in Fig. 61 and described in the section on axial 

 gradients. 



Class Trematoda 



The trematodes or flukes are exclusively para- 



. . ,. . . , , ., Fig. 62.— a 



sitic, living either as ectoparasites upon or as r h a b d o c o e i 

 endoparasites within the bodies of various ani- (Microstomum) 



, „. .... . . 1 J , in the process of 



mais. bmce parasitism is an acquired and not a fission into six- 

 primitive mode of life, the bodies of all trematodes teen zooids. 



1 1 • f 1 • X r Roman numer- 



are more or less modmcd as an accompaniment oi ^is indicate the 

 the parasitic habit. Simplicity of structure in this order of the 



, 1 , , 1 . ■ -,■ ■ fission planes. 



class denotes degeneracy and not primitive sim- (After von Graff). 

 plicity. While there are no free-living forms, 

 most trematodes in their fundamental structure present clear 

 evidences of close relationship with the Turbellaria. They vary 

 from less than one millimeter to several centimeters in length. 

 A cuticula, frequently supplied with spines, covers the body 

 surface of all adult trematodes, while some larval forms (Fig. 



